1. Field of the Invention
The present invention relates to the preparation of linear polychlorophosphazenes having a terminal -PXCl.sub.2 group, in which X is an oxygen or sulfur atom, by polycondensation of N-dichlorophosphoryl- or N-dichlorothiophosphoryl-P-trichlorophosphazene (P.sub.2 NXCl.sub.5).
2. Description of the Prior Art
It is known to this art that linear polychlorophosphazenes having a terminal --PXCl.sub.2 group are formed by heating the compounds of the formula P.sub.2 NXCl.sub.5 under conditions of pressure and temperature suited to the release of PXCl.sub.3.
Thus, French Patent No. 79/24,037 (publication No. 2,466,435) describes the preparation of linear polychlorophosphazenes which have a terminal dichlorophosphoryl group of the formula --POCl.sub.2, by polycondensing the compound P-trichloro-N-dichlorophosphorylmonophosphazene of empirical formula P.sub.2 NOCl.sub.5 under conditions of pressure and temperature selected to release POCl.sub.3.
French Patent Application No. 83/11,264 (publication No. 2,548,652) describes the preparation of linear polychlorophosphazenes which have a terminal dichlorothiophosphoryl group of the formula -PSCl.sub.2, by polycondensing the compound P-trichloro-N-dichlorothiophosphorylmonophosphazene of empirical formula P.sub.2 NSCl.sub.5 under conditions of pressure and temperature conditions which are suited to the release of PSCl.sub.3.
These linear polychlorophosphazene having a terminal --PXC.sub.2 group correspond to the following empirical formula: EQU Cl.sub.2 (X)P--NPCl.sub.2 --.sub.n Cl (I)
in which n is a number equal to or greater than 4, and wherein the maximum value of n can be 5,000 or even more.
Despite undoubted advantages, the most notable of which being its simplicity, the high degree of conversion of the monomer and the relatively low cost of such monomer, the above-mentioned process for the preparation of linear polychlorophosphazenes having a terminal --PXCl.sub.2 group by thermal polycondensation of the monomer P.sub.2 NXCl.sub.5 is not wholly satisfactory, because it does not permit reproducibly attaining high degrees of polycondensation.
Indeed, when the evolution of PXCl.sub.3 ceases, the average degree of polycondensation of the resulting linear polychlorophosphazenes is low, namely, n is on the order of 20 to 30; this indicates that said polychlorophosphazenes are short-chain polymers. To obtain a polycondensate which has a higher degree of polycondensation, these short-chain polymers must react together via their chain ends to form long-chain polymers, and this mandates continuing the polycondensation after the evolution of the compound PXCl.sub.3 has ceased. The progress of this second stage of polycondensation is difficult to control and obtaining uncrosslinked polychlorophosphazenes which have degrees of polycondensation that are higher than approximately 500 remains a random process. In fact, the risk of crosslinking increases with the increase in the length of the polymer chains, and this phenomenon develops in a manner which can vary from one test to another. This results, therefore, in a mediocre reproducibility when attempting to attain high degrees of polycondensation. Such constitutes a severe handicap for an industrial application of the bulk process, because most of the applications of linear polychlorophosphazenes require polymers which have high degrees of polycondensation.
Moreover, this technique of polycondensation is generally carried out in a stirred reactor of a common type, comprising a vessel heated by elements in the wall members thereof and being equipped with an anchor stirrer. With this apparatus, the bulk polycondensation must be terminated at a molecular weight value which is limited at most to about 100,000, at the risk of crosslinking and producing an insoluble and unusable polymer.
French Patent No. 84/15,892 (publication No. 2,571,710) describes carrying out the polycondensation, either wholly or during its final stage, in solution. This improvement imparts to the polycondensate a great resistance to crosslinking phenomena and makes it possible to obtain a linear polydichlorophosphazene having a high molecular weight.
However, this latter process presents a certain number of disadvantages associated with the use of solvents. Indeed, such solvents must be of high purity and carefully dehydrated. At the very high temperatures at which the polycondensation is carried out, the presence of a solvent makes it necessary to conduct the operation under pressure. The combination of the high temperatures, of the pressure and of a highly corrosive environment presents equipment problems which are difficult to solve. The fact of operating in solution results in additional cost of the reactor, which must be larger in size and which must withstand pressure. Finally, the solvent must be separated from the polymer and purified, to be recycled.